1. Field of the Invention
The present invention relates to a wireless communication device with an improved antenna structure, and more particularly to a wireless communication device with an improved antenna structure suitable for a high radiation efficiency.
2. Description of the Related Art
The wireless device of mobile type has an antenna for radiating and receiving radio-waves. FIG. 1 is a schematic perspective view of a mobile telephone with a whip antenna. A mobile telephone 1 has a whip antenna 3 which may be extendable and retractable. In place of the whip antenna 3, a helical antenna fixed to a case of the mobile telephone 1 may alternatively be provided. Further, the antenna may be integrated into the case of the mobile telephone 1.
The mobile wireless device has a ground which comprises an electrically conductive plate, wherein a power is supplied into between the ground and the antenna, whereby an electric image is generated. on the ground in symmetrical to an electric image on the antenna 3. The antenna may comprise a dipole antenna which has a length approximately equal to one quarter of the transmitting radio wave. A shield in the case of the mobile wireless device, a shield covering circuitries or circuit parts, and a ground pattern of a printed board may act as the ground. A high frequency current flows on a surface of the conductor as the ground. Performances of the antenna may be an input impedance and a radiation efficiency. The performances of the antenna depend on the current flowing on the ground plate.
The mobile telephone device may perform a transmission operation at a high frequency in the vicinity of 1 HGz. A wavelength of the radio wave is approximately 30 cm, and a quarter-wavelength of the radio wave is thus approximately 7.5 cm. The mobile telephone device is scaled down so that a width is, for example, about 4 cm which is narrower than the quarter-wavelength.
Recently developed mobile telephone devices may perform transmission operations at a higher frequency near 2 GHz, wherein the wavelength of the radio wave is approximately 15 cm, and the quarter-wavelength of the radio wave is thus, slightly shorter than 4 cm. The width of the case of the mobile telephone is about 4 cm which is approximately equal to the quarter-wavelength of the radio wave.
As described above, the high frequency current flowing through the case of the mobile telephone generates the electric image on the case of the mobile telephone. FIG. 2 is a schematic view illustrative of a model of the applied high frequency current on the ground plate of the conventional mobile telephone. The antenna 3 is provided at a feeding point which is positioned at a center point on a top side of the ground plate 2. A first current 5a and a second current 5b from the feeding point on the one side including the feeding point 4 have the same phase and opposite directions, wherein fields generated. by the first and second currents 5a and 5b are canceled with each other, whereby effective high frequency currents, which generate the electric image, do not appear in a macroscopic view. As a result, the electric image to the antenna 3 is not generated. on the ground plate 2. This means that the provision of the antenna element 3 in the vicinity of the center position on one side of the ground plate 2 makes the radiation efficiency deteriorated.
In the above circumstances, the development of a novel wireless device with an improved antenna free from the above problems is desirable.